1. Field of the Invention
The present invention relates to a mechanism usable for providing a channel feedback in a multi-cell communication scenario. In particular, the present invention is related to a method, apparatus and computer program product providing a mechanism allowing a compressed multi-cell channel state information feedback.
2. Related Background Art
Prior art which is related to this technical field can e.g. be found by the technical specification 3GPP TS 36.213, v.9.3.0.
The following meanings for the abbreviations used in this specification apply:                3GPP—3rd generation partnership project        BLER—block error rate        CoMP—coordinated multi-point (transmission or reception)        CQI—channel quality indicator        CRC—cyclic redundancy check        CSI—channel state information        CSI-RS—channel state information reference symbols        DL—downlink        eNB—enhanced Node B (LTE based base transceiver station)        FDPS—frequency domain packet scheduling        IP—Internet protocol        LTE—long term evolution        MCS—modulation and coding scheme        MIMO—multiple input multiple output        PMI—precoding matrix indicator        PRB—physical resource block        RI—rank indicator        RS—reference symbol        SINR—signal to interference noise ratio        TBS—transport block size        TX—transmission        UE—user equipment        UL—uplink        
In the last years, an increasing extension of communication networks, e.g. of wire based communication networks, such as the Integrated Services Digital Network (ISDN), DSL, or wireless communication networks, such as the cdma2000 (code division multiple access) system, cellular 3rd generation (3G) communication networks like the Universal Mobile Telecommunications System (UMTS), enhanced communication networks based e.g. on LTE, cellular 2nd generation (2G) communication networks like the Global System for Mobile communications (GSM), the General Packet Radio System (GPRS), the Enhanced Data Rates for Global Evolutions (EDGE), or other wireless communication system, such as the Wireless Local Area Network (WLAN) or Worldwide Interoperability for Microwave Access (WiMAX), took place all over the world. Various organizations, such as the 3rd Generation Partnership Project (3GPP), Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), the IEEE (Institute of Electrical and Electronics Engineers), the WiMAX Forum and the like are working on standards for telecommunication network and access environments.
A next step or evolution in LTE based systems will be so-called LTE-Advanced which shall fulfill further requirements. One of the study items is coordinated multi-point transmission/reception or CoMP.
Generally, for properly establishing and handling a communication connection between network elements such as a UE and another communication equipment, such as another UE, a server, etc., one or more intermediate network elements, such as base stations or access nodes, network control nodes, support nodes, service nodes and interworking elements are involved which may belong to different communication networks.
In a CoMP scenario, the situation is further enhanced. For example, CoMP a method of transmitting to or receiving from a UE using several base stations. The general goal is to improve data throughput. Basically, in a CoMP communication scenario, inter-cell interferences are tried to be turned into useful signal, especially at a cell border where performance may be degraded. While high data rates are relatively easy to maintain close to a base station, when distances increase, they become more difficult to maintain. Not only is the signal lower in strength because of the distance from the base station such as an eNB, but also interference levels from neighboring eNBs are likely to be higher as the UE will be closer to them.
Therefore, it is intended in a CoMP scenario that a close coordination between a number of geographically separated eNBs is achieved. The eNBs may dynamically coordinate to provide joint scheduling and transmissions as well as proving joint processing of the received signals. In this way a UE at the edge of a cell is able to be served by two or more eNBs to improve signals reception/transmission and increase throughput particularly under cell edge conditions.
In essence, CoMP may provide two modes of operation:                Joint simultaneous transmission of user data from multiple eNBs to a single UE        Dynamic cell selection with data transmission from one eNB.        
To achieve either of these modes, highly detailed feedback is required, for example on channel properties, in a fast manner so that necessary changes can be made. In other words, in order to make CoMP possible, it is necessary to provide, for example, an accurate Chanel State Information (CSI) Feedback which is sent from the UE to the (plural) eNB(s).
In order to achieve gains from techniques for improving the system capacity, such a frequency dependent packet scheduling (FDPS), it is necessary to obtain accurate information about a propagation channel, such as frequency domain behavior of the propagation channel. For example, for enabling use of FDPS, several frequency selective CSI reporting modes are proposed and also included in specifications of e.g. LTE.
However, the size of CSI reports depends on several elements, such as the used reporting mode, a system bandwidth and a rank. For example, in existing solutions considering a connection between one UE and one eNB (i.e. a non-CoMP case), the maximum size of a CSI report may be 64 bits (as so-called reporting mode 3-1, bandwidth is 20 MHz, rank>1), without CRC bits.
As indicated above, when a communication scenario using CoMP is established, e.g. a DL CoMP operation, a UE is simultaneously connected to multiple eNBs. When sufficient and accurate CSI reports are available, the cooperating eNBs are able to e.g. select optimal precoders and frequency resources for data transmission (as in coordinated scheduling/beamforming) or perform joint transmission to the UE simultaneously from multiple points. However, in order to achieve this, all CoMP schemes requires such accurate CSI reports for all the cooperating cells, on which basis the eNBs can then conduct a communication control and choose how and which of the different cells interact.
In order to provide such sufficient and accurate CSI reports, there are conceivable several options. One straight forward option would be to simply provide separate frequency selective CSI reports for each of the cooperating cells, i.e. to treat every cell connection in the CoMP scenario as if it is the only connected cell. However, in such a case, the resulting procedure is typically not feasible, for example since report sizes would probably become excessive. For example, assuming a case where three cells with 4 TX antennas each are communicating with an UE. When using, for example, a report mode 3-1, a resulting CSI reporting would require transmission of 3*64 bits=192 bits (excluding the additional CRC bits) which represents a rather high overhead.
For example, a corresponding CSI reporting mode which is already discussed in the LTE related specifications, such as in LTE Release-8, is the so-called Mode 2-2, also known as Best-M average. In this Mode 2-2 the UE connected to one eNB reports:                One CQI value for each codeword reflecting the transmission over the M best subbands (a group of consecutive PRBs, M>0), which has a size of e.g. 4+2 bits        One precoding matrix preferred for the M selected subbands, which has a size of e.g. 4 bits for cells with 4 TX antennas        One precoding matrix preferred for all the subbands, which has a size of e.g. 4 bits for cells with 4 TX antennas        One CQI value for each codeword reflecting the transmission on all the subbands, which has a size of e.g. 4+2 bits        An indication of the M preferred subbands, which may have a size of 18 bits for a 20 MHZ system bandwidth        Additionally, the UE may also report a separately encoded RI.        
In summary, when a system configuration with an eNB having 4 TX antennas and a 20 MHz system bandwidth is assumed, a resulting payload size of a report based on the CSI reporting Mode 2-2 is 38 bits. However, as indicated above, when such an aperiodic CSI Mode 2-2 is used in case of a CoMP scenario with more than one eNB to be measured and reported to, the payload size becomes large, for example 3*38 bits with three cells. However, since UEs using CoMP may be typically close to a cell border and are hence often UL coverage limited, payload is a critical issue. Furthermore, the CSI reporting Mode 2-2 as discussed in the existing LTE specifications and described above is not really suitable for a multi-cell operation such as CoMP. The reason is that typically the best-M subbands the UE identifies are not the same for different cells which complicates a processing of an eNB scheduler to figure out which part of the spectrum is to allocate to the reporting UE.
As another option, it is also possible to just rely on a wideband feedback, i.e. a frequency non-selective feedback for CoMP. This option would have the advantage of producing only a small overhead, for example 11 bit/cell or less. However, this option is also not optimal since in such a case it would not be possible to achieve certain benefits of capacity improving techniques such as FDPS, which may be in a range of approximately 40% in throughput, depending on the conditions.